Method of producing multilayer plate for x-ray imaging

ABSTRACT

A method for producing a multilayer plate for X-ray imaging is disclosed. As the first step in the method, there is provided a plate of a substrate. Then, there is deposited on this substrate a thin film of amorphous arsenic triselenide by thermal evaporation under reduced pressure, followed by condensation on the substrate. A thick photoconductive film of doped amorphous selenium is then deposited by evaporation and condensation on the thin layer of amorphous arsenic triselenide. This thick photoconductive film can also be deposited directly onto the substrate. Then, a thin film of alkali doped selenium is deposited onto the thick photoconductive layer by evaporation or co-evaporation and condensation, and a conducting biasing electrode is formed on top of this alkali doped film.

This is a divisional application of application Ser. No. 08/827,512filed on Mar. 28, 1997, now U.S. Pat. No. 5,880,472.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to an improved multilayer plate for X-rayimaging and to a method for producing such plate, used for convertingX-rays into a latent electrostatic image. This latent electrostaticimage can subsequently be read out by various schemes, such as by ascanning laser beam, a microcapacitor active matrix panel, or a bank ofelectrostatic probes.

2. Description of the Prior

It is already known to produce multilayer X-ray imaging plates which aresometimes referred to as xeroradiographic plates.

For example, U.S. Pat. No. 3,975,635 of Aug. 17, 1976 discloses axeroradiographic plate consisting of a conductive backing having thereona photoconductive layer of selenium and an intermediate layer of analloy comprising about 15-45 wt % of arsenic and 55-85 wt % of selenium,which intermediate layer has a thickness of about 15-150 μm and is usedto reduce the capacitance of the structure with the result that imagesare obtained which are capable of development at lower fields withoutsubstantial loss of resolution.

U.S. Pat. No. 4,286,033 of Aug. 25, 1981 discloses a multilayerinorganic photosensitive device which comprises a number of variouslayers, one of which is a hole trapping layer consisting of a halogendoped selenium arsenic alloy wherein the amount of selenium ranges from95-99.9 wt %, the amount of arsenic ranges from 0.1 to 5 wt % and theamount of halogen is from 10-200 ppm (parts per million). This holetrapping layer has a thickness of 0.01-5 μm (microns), and is used toretain positive charges at the interface between the generating layerand the overcoating insulating layer, thereby improving image quality.

U.S. Pat. No. 4,338,387 of Jul. 6, 1982 relates to an overcoatedphotoreceptive device containing a layer of electron trapping materialand a hole trapping layer, these layers being comprised of a halogendoped selenium arsenic alloy wherein the amount of selenium is about95-99.9 wt %, the amount of arsenic is between 0.1-5 wt % and the amountof halogen is from 10 ppm to 200 ppm.

U.S. Pat. No. 4,770,965 of Sep. 13, 1988 discloses a selenium alloyimaging member suitable for X-ray imaging, which is characterized byproviding on the Se alloy layer a thin protective organic overcoatinglayer having about 0.5-3 wt % of nigrosine. This is claimed to result ina greater resolution at a significantly reduced X-ray dosage. In thisU.S. Pat. No. 4,770,965, the concept of using intermediate polymeradhesive primer layers between the selenium layer and the metal oxidesurface is also disclosed. However, these polymer layers have highthermal expansion coefficients and are not effective in reducing theshear stress due to different thermal expansion of the various layers inthe device and may result in film cracking.

In U.S. Pat. No. 4,891,290 of Jan. 2, 1990 there is disclosed amultilayer photosensitive material for electrophotography, (rather thanx-ray imaging) wherein a high surface hardness is obtained by providinga surface protective layer of an arsenic-selenium alloy having acomposition of approximately As₂Se₃. Such photosensitive material has ahigh printing resistance. It is also indicated that such photosensitivematerial may include a buffer layer comprising an arsenic-selenium alloydisposed between the surface protection layer and the charge generationlayer which allows for high temperature operation. It should be notedthat in electrophotography, to which this U.S. patent relates, the tonerparticles are mechanically cleaned between images, whereas in digitalX-ray imaging there is no mechanical abrasion of the surface and thus ahigh surface hardness in not required.

In U.S. Pat. No. 4,990,419 of Feb. 5, 1991 assigned to Fuji Electric Co.Ltd., a multilayer electrophotographic photoreceptor is again disclosed,which comprises an As₂Se₃ carrier transport layer, a 30 to 50 wt % Te—Sealloy carrier generation layer and an As₂Se₃ surface protection layer aswell as an outer layer of a transparent insulating material and in U.S.Pat. No. 5,021,310 of Jun. 4, 1991 also assigned to Fuji Electric Co.Ltd. a further thermal expansion relieving layer comprising arsenic andselenium is provided within the photoreceptor. It is stated in thispatent that the As concentration of the thermal expansion relievinglayer varied from 10 wt % to 38.7 wt % and its overall thickness was 1μm. A surface protective layer of As₂Se₃ containing 1000 ppm of iodinewas deposited thereon to a thickness of 3 μm. Again, this patent relatesto an electrophotographic photoreceptor, rather than to an X-ray imagingdevice.

According to U.S. Pat. No. 5,023,661 of Jun. 11, 1991, it has beendetermined that a fatigue artifact is caused by a defect in thexeroradiographic plate in the form of a selenium crystallite at thelower surface of the selenium layer of the plate, which allows positivecharges in the form of holes, to enter the selenium layer from thealuminum base during the transfer step. These are often called“catastrophic spot producing artifacts”, and the U.S. patent provides aprocess for eliminating such artifacts by pre-charging the detectorafter a thermal relaxation step to eliminate the trapped space charge inthe device.

In U.S. Pat. No. 5,320,927 of Jun. 14, 1994 the technology formanufacturing an improved selenium alloy X-ray imaging member on atransparent substrate is examined, wherein a bulk selenium arsenicmaterial containing 0.1 to 0.6 wt % As is evaporated onto said substratein a controlled fractionation process and the evaporation isdiscontinued when the weight of the selenium alloy remaining in the boatis 2-10% of the original weight. This patent also teaches the use of aselenium arsenic alloy (1-24% As) between the X-ray absorbing materialand the substrate material to reduce the crystallite-induced defects.However, this patent fails to address the issue of mechanical stabilityof the photoreceptor as well as the space charge neutralizationcapability of the structure.

In U.S. Pat. No. 5,330,863 of Jul. 19, 1994 a photosensitive materialfor use in electric photography is disclosed wherein carrier injectionpreventing layers consisting of selenium/arsenic/sulphur alloy areinserted between the conductive substrate and the carrier transportlayer or between the carrier generation layer and the overcoat layer, orbetween both. This makes the photosensitive material resistant tofriction, heat, dark decay and fatigue and exhibits little deteriorationunder high temperature environments. This patent does not relate toX-ray imaging.

In U.S. Pat. No. 5,396,072 of Mar. 7, 1995 a fairly complex X-ray imagedetector is disclosed, which comprises a plurality of X-ray sensitivesensors each of which has a collecting electrode, a reference electrodeand a switching element which connects the collecting electrode to anoutput lead; a photoconductor layer is provided between the individualcollecting electrodes and a bias electrode; and each of the collectingelectrodes comprises two electrically contacting electrode portionsarranged and situated in a very specific manner, so that the majority ofthe charge carriers generated in the photoconductor flow to thecollecting electrodes.

In U.S. Pat. No. 5,436,101 of Jul. 25, 1995 an X-ray photoreceptor isdisclosed which has a high arsenic interstitial layer 5-40 μm inthickness sandwiched between the substrate and the selenium layer fortrapping positive charges injected from the interface. This structurewas designed to prevent rather than promote hole injection from thesubstrate material into the photoreceptor device.

It should be noted that the concept of using multilayer structures basedon amorphous selenium alloys (a-Se alloys) originated in theelectrophotographic or xerographic industry (see, for example, U.S. Pat.No. 3,041,166 of Jun. 26, 1962) in an effort to make the spectralresponse of the photoreceptor more panchromatic to compete with thelower cost organic photoreceptors. For example, alloying of Se withabout 40 atomic % Te has been shown to decrease the effective opticalband gap of selenium from 2.2 eV down to about 1.2 eV. However, thisincreased longer wavelength photosensitivity generally occurs at theexpense of electrophotographic properties—high residual potentials andrapid dark decays being typical of this class of materials. In fact, theelectrophotographic properties of a-Se₅Te_(1-x), materials, particularlywhen the Te content is high, generally preclude the use of thesematerials in monolayer photoreceptor applications. Since photoreceptorsrequire both low residuals, wide panchromicity (especially for laserprinter applications, where low cost semiconductor lasers emit light inthe long wavelength regime), and low dark decay, considerable effort wasplaced into decoupling the photogeneration process and the chargetransport process in the device. Se_(x)Te_(1-x), alloys were used toabsorb the light, but since the xerographic properties of this materialwere not optimal, a second charge transport layer was used to achievethe desired electrophotographic properties.

As is obvious from the various prior art patents referred to above,multilayer selenium based structures have also been employed for higherenergy X-ray imaging applications. One of the earliest commercialapplications of selenium to X-ray imaging was in xeroradiography, wherethe detector consisted of a selenium layer deposited onto an aluminumplate. In a typical imaging cycle, the plate was sensitized by coronacharging, exposed to the patient modulated X-ray beam to selectivelydischarge the selenium, and then developed by passing triboelectricallycharged toner particles across the selenium plate, transferring thetoner particles to paper, and finally fixing the image by heating thepaper. Before the next image could be taken, the selenium plate had tobe cleaned from all residual toner particles (generally by mechanicalbrush), and then restored to a “neutral space charge” condition. Themultilayer structures used in optical imaging applications and thoseused in X-ray imaging applications are not interchangeable and haveacquired separate status within the relevant art as is obvious from theprior art patents discussed above.

Furthermore, within the X-ray imaging itself there are two distinctmodes of imaging, namely the static mode and the dynamic mode which maybe defined as follows:

Static Mode Imaging

In the static mode imaging, images can only be taken at a relatively lowfrequency, e.g. 1 image every 20 seconds, and the X-ray beam is pulsed.As such, there is sufficient time to neutralize any space charge whichaccumulates in the device between images.

Dynamic Mode Imaging

In the dynamic mode imaging, images are taken at a much higherfrequency, e.g. 30 images per second, and the X-ray beam is left onduring the entire examination. In this case, there is no time to removethe applied bias voltage between images to allow holes to be injectedfrom the bottom buffer layer into the bulk X-ray absorbing layer toneutralize the negative space charge.

Although the above discussed prior art indicates that a considerableamount of work is being done in the area of optical and X-ray imagingtechnologies, until now, selenium based X-ray detectors have sufferedfrom the presence of polycrystallites in the selenium layer located nearthe substrate. The presence of such polycrystallites is undesirable inX-ray imaging applications, since it could lead to spurious chargeinjection sites and in the extreme case to a loss of the imagingcapabilities for X-ray imaging detectors where the latent electrostaticimage is read from the substrate. The manufacturing process window forproducing a layer which is free of polycrystallites at the interfacewhile simultaneously keeping the bulk properties of the amorphousselenium layer at their optimal value is extremely narrow.

Furthermore, until now selenium-based X-ray detectors have suffered fromthermal shocks which often lead to the physical delamination of theselenium film from the substrate due to the stress resulting from themismatched thermal expansion between the bulk amorphous selenium layerand typical substrate materials such as glass and aluminum.

Moreover, prior art selenium based X-ray detectors have suffered fromthe availability of a limited number of materials which could be used asthe substrate electrode material. For example, aluminum has been widelyused because of its high oxidation potential and hence its ability toform a high-quality uniform aluminum oxide layer to prevent electroninjection from the substrate into the bulk of selenium. Another exampleis Indium Tin Oxide (ITO) coated glass which has shown some electronblocking characteristics at the ITO selenium heterojunction.

However, known detectors do not normally allow the use of a wide varietyof substrate materials because they rely on the electrochemicalinteraction between the materials to create the required electronblocking characteristic.

In addition, prior art selenium-based X-ray detectors have suffered frommemory effects induced by the accumulation of negative space charge inthe doped selenium layer. Laborious erasing schemes utilizing acombination of light, temperature and voltage polarization cycles werenecessary to erase the accumulated space charge. In the case of opaquesubstrate materials, this prohibits the use of light in the erasuresequence.

Finally, known selenium-based X-ray detectors have suffered fromdifficulties in applying the high voltage bias across the dopedamorphous selenium layer. This problem was handled by either coronacharging the device or by inserting insulating materials such aspolycarbonate, polyester, parylene or glass between an upper electrodeand the doped amorphous selenium layer to prevent spurious holeinjection from the electrode into the selenium layer. None of theseapproaches allow for imaging at fluoroscopic rates (30 images/second).

OBJECTS AND SUMMARY OF THE INVENTION

It is an object of the present invention to provide a multilayer platefor X-ray imaging and a method of its manufacture which will obviate thedisadvantages of the prior art.

Another object of the invention is to provide an improved X-ray imagingmultilayer membrane which can be used in a variety of X-ray imagingtechnologies, including medical and non-destructive testingapplications.

Other objects and advantages of this invention will become apparent fromthe following description thereof.

In essence, the multilayer plate for X-ray imaging in accordance withthe present invention comprises:

(a) a substrate, which may be of any desired type;

(b) a biasing electrode, which also may be of any suitable type; and

(c) a selenium based membrane sandwiched between said substrate and saidbiasing electrode, said membrane comprising a thin interstitial bufferlayer of amorphous arsenic triselenide which will normally have athickness of between about 0.5 μm and 10 μm, preferably between about 1μm and 5 μm, and a thick photoconductive layer of doped amorphousselenium which will normally have a thickness of between about 100 μmand 2 mm, preferably between about 200 μm and 500 μm, said interstitialbuffer layer being itself sandwiched between said substrate and saidphotoconductive layer.

The interstitial buffer layer mentioned above is essential for thestatic mode X-ray imaging because the charge neutralization in this modeis accomplished by the injection of holes from the bottom interstitialbuffer layer into the X-ray absorbing photoconductive layer when theapplied bias voltage is removed. This amorphous arsenic triselenide(a-As₂Se₃) bottom layer allows “self-reconditioning” of the plate andhas other intrinsic advantages, such as being electron blocking andadhesion promoting during the thermal cycling of the device.

In another embodiment of the invention, there is provided a multlayerplate for X-ray imaging, which comprises:

(a) a substrate, which again may be of any desired type;

(b) a biasing electrode, which will normally consist of a thin layer ofconductive material, such as a metal film; and

(c) a selenium based membrane sandwiched between said substrate and saidbiasing electrode, said membrane comprising a thin unipolar conductingbuffer layer made of alkali doped selenium, which will normally have athickness of between about 0.5 μm and 10 μm, preferably between about 1μm and 5 μm, and a thick photoconductive layer of doped amorphousselenium which will normally have a thickness of between about 100 μmand 2 mm, preferably between about 200 μm and 500 μm, said unipolarconducting buffer layer being itself sandwiched between said biasingelectrode and said photoconductive layer.

The unipolar conducting buffer layer mentioned above is essential forthe dynamic mode imaging because it is designed to minimize any excesscharge injection from the top electrode into the bulk material until thecharge carriers generated by the X-ray beam neutralize the space chargein the device. In such dynamic mode, the biasing electrode will notcomprise a layer of insulating dielectric material as in the case of thestatic mode, but will consist solely of a thin layer of conductivematerial applied over the unipolar conductive buffer layer. It ispreferred that also in this case there be provided the interstitialbottom buffer layer of amorphous arsenic triselenide already mentionedabove to impart the additional advantages, such as blocking of electronsand buffering the differential thermal expansion between the substrateand the bulk absorbing layer.

Thus, according to the most preferred embodiment of the presentinvention, there is provided a multilayer plate for X-ray imaging, whichcomprises:

(a) a substrate, which again may be of any desired type;

(b) a biasing electrode, which also may be of any suitable type; and

(c) a selenium based membrane sandwiched between said substrate and saidbiasing electrode, said membrane essentially consisting of a thininterstitial buffer layer of amorphous arsenic triselenide, a thickphotoconductive layer made of doped amorphous selenium and a thinunipolar conducting buffer layer made of alkali doped selenium, saidphotoconductive layer being itself sandwiched between said buffer layerswith the interstitial buffer layer being positioned between saidphotoconductive layer and the substrate, while the unipolar conductingbuffer layer being positioned between said photoconductive layer and thebiasing electrode.

The layered plate structures of the present invention allow conversionof X-rays into a latent electrostatic image that can subsequently beread out by various schemes. For example, such image can be read by ascanning laser beam, a microcapacitor active matrix panel or a bank ofelectrostatic probes.

In the preferred embodiment mentioned above, the doped amorphousselenium layer is used to absorb and convert the incident X-ray energyinto electrical charges, whereas the buffer layers are used to increasethe compatibility of the structure to a wide variety of detectorconfigurations, thereby making this invention generic to any directconversion X-ray imaging systems.

The present invention also includes a method of manufacturing an X-rayimaging plate, which comprises:

(a) providing a substrate;

(b) depositing on said substrate a thin film of amorphous arsenictriselenide by thermally evaporating doped arsenic triselenide materialunder reduced pressure of less than 1×10⁻⁵ torr and condensing theresulting vapour onto the substrate to form a uniform amorphous layer ofAs₂Se₃;

(c) depositing on said thin film of amorphous arsenic triselenide athick photoconductive film of doped amorphous selenium by evaporating adoped amorphous selenium material and condensing the resulting vapouronto said thin film of amorphous arsenic triselenide; and

(d) laminating or coating onto the thick photoconductive film aninsulating dielectric layer and providing on top of said insulatingdielectric layer a thin layer of conductive material, said insulatingdielectric layer and said layer of conductive material constituting abiasing electrode.

In a preferred embodiment, the method further comprises depositing onsaid photoconductive film a thin film of alkali doped selenium byevaporating an alkali doped selenium alloy or simultaneouslyco-evaporating Se and alkali material and condensing the resultingvapour onto the photoconductive film, and thereafter forming the biasingelectrode on top of said thin film of alkali doped selenium.

If, in a dynamic mode imaging situation, the bottom thin film ofa-As₂Se₃ is not required, one may only deposit the alkali doped seleniumlayer onto the photoconductive film and thereafter form the biasingelectrode on top of said layer, which will consist of a thin layer ofconductive material, such as a metal film.

When reference is made to the thin interstitial buffer layer or film ofamorphous arsenic triselenide, it should be understood that it isnormally formed by thermally evaporating doped arsenic triselenidematerial the composition of which contains about 34-38 wt % As andincludes dopants such as iodine, indium or gallium in parts per millionconcentrations, and thus the a-As₂Se₃ film also normally contains suchdopants.

The second thin buffer layer or thin film referred to as a unipolarconducting buffer layer also may include arsenic in the 0.5-5 wt % rangeas well as an alkali element, such as Li, K, Na and H in the 1-1000 ppmconcentration or some combination thereof.

The thick photoconductive layer of doped amorphous selenium isconventional and is normally made of amorphous selenium doped witharsenic and chlorine. For example, it may contain 0.2% As and 10 ppm Cl.

The amorphous arsenic triselenide interstitial buffer layer used inaccordance with the present invention provides a wide process windowinasmuch as the tendency for a-As₂Se₃ layer to crystallize is much lessthan that of the doped selenium layer. Moreover, the arsenic triselenidebuffer layer has the ability to reduce the stress resulting from themismatched thermal expansion between the bulk amorphous selenium layerand typical substrate materials such as glass and aluminum. Furthermore,the amorphous arsenic triselenide buffer layer allows a wide variety ofsubstrate materials to be used since it divorces the electronicproperties of the substrate from the doped selenium layer and does notrely on the electrochemical interaction between the materials to createthe electron blocking characteristic, because of the inability ofelectrons to traverse even a very thin (0.5-10 μm) layer of a-As₂Se₃biased to high electric fields in excess of 15 V/μm.

Another important feature of this invention is the ability of theamorphous arsenic triselenide layer to inject a sufficient amount ofpositive space charge into the doped amorphous selenium layer to restorespace charge neutrality when the applied bias is removed from thedetector.

A still further key element of this invention is to provide a holeblocking layer also called a unipolar conducting buffer layer directlydeposited on the doped amorphous selenium layer prior to deposition ofany conducting metal electrode such as indium, gold, aluminum, chromiumor ITO (indium tin oxide). This hole blocking layer must be unipolar innature for trapping holes injected from the upper electrode, butconducting electrons generated from X-rays in the doped amorphousselenium layer to prevent the accumulation of a negative space charge inthe device.

As already mentioned above, in the most preferred embodiment of theinvention, both of the above described buffer layers are used within thex-ray imaging plate.

BRIEF DESCRIPTION OF THE DRAWINGS

A preferred embodiment of the invention will now be described withreference to the appended drawings in which:

FIG. 1 is a cross-sectional view of the multilayer X-ray imaging plateon an enlarged scale;

FIG. 2(a) is a cross-sectional view of an experimental prototype usedfor comparison purposes;

FIG. 2(b) is a cross-sectional view of another experimental prototypeused for comparison purposes;

FIGS. 3(a), 3(b), 3(c), 3(d), and 3(e) illustrate various failuremechanisms at the selenium substrate interface when it is stressed;

FIG. 4 is a comparative plot of plate stress versus plate temperaturefor devices such as illustrated in FIG. 2(a);

FIG. 5(a) illustrates hole transient photoconductivity waveforms of aconventional X-ray imaging plate;

FIG. 5(b) illustrates comparative hole transient photoconductivitywaveforms of an X-ray imaging plate according to the present invention;

FIG. 6(a) illustrates photoinduced discharge measurements showing theelectron blocking of a conventional X-ray imaging plate;

FIG. 6(b) illustrates comparative photoinduced discharge measurements todemonstrate the electron blocking efficiency of the novel X-ray imagingplate;

FIG. 7 is a comparative plot of dark current versus voltagecharacteristics for a device illustrated in FIG. 2(b);

FIG. 8(a) illustrates the total current as a function of time for adevice such as shown in FIG. 2(b) without buffer layer 22; and

FIG. 8(b) illustrates the total current as a function of time for adevice such as shown in FIG. 2(b) with buffer layer 22.

DETAILED DESCRIPTION OF THE INVENTION

In FIG. 1 the most preferred embodiment of the present invention isillustrated. It shows a multilayered plate 10 which comprises asubstrate 12 which, in this case, is shown to be a TFT matrix (thin filmtransistor), and a biasing electrode 14 which is made of a high voltagebiasing structure capable of withstanding voltages in excess of 500volts. A selenium based multilayered membrane 16 is sandwiched betweenthe substrate 12 and the biasing electrode 14. The substrate 12 can beany desired substrate, such as aluminum, glass, a thin film transistorarray, a charged coupled device (CCD) and a complementary metal oxidesemiconductor device (CMOS).

In accordance with the preferred embodiment of the present inventionillustrated in FIG. 1, this membrane comprises an energy absorbing andconverting layer 18 which is also called a photoconductive layer andwhich is made of doped amorphous selenium. Layer 18 is a thick filmnormally having a thickness of between about 100 μm and 2 mm, preferablybetween about 200 μm and 500 μm, and is generally known in the art. Thislayer 18 is itself sandwiched between two thin buffer layers 20 and 22(usually between about 0.5 μm and 10 μm thick) which are the gist of thepresent invention.

As already mentioned, the invention could include only buffer layer 20or only buffer layer 22 within the structure such as shown in FIG. 1,but in the most preferred embodiment it includes both of these bufferlayers.

Finally, there is provided an electrical connection 24 between thesubstrate 12 and the biasing electrode 14 to impart the required highvoltage during the operation of the device.

The doped amorphous selenium layer will have properties, such as X-rayabsorption, charge generation, charge transport and dark dischargeoptimized to suit a given X-ray imaging requirement. The amorphousarsenic triselenide (a-As₂Se₃) interstitial buffer layer 20 sandwichedbetween the doped amorphous selenium layer 18 and the substrate 12 wassurprisingly found to have properties that make the structure of thenovel multilayer plate mechanically stable by promoting strong adhesionbetween the substrate 12 and the selenium layer 18. This buffer layeralso allows the structure to recondition itself without difficulty and,moreover, significantly increases the manufacturing process window inwhich crystallization is avoided.

The second buffer layer 22, sandwiched between the amorphous seleniumlayer 18 and the biasing structure 14, is a unipolar conducting bufferlayer made of alkali doped selenium. This layer 22 is designed such thatthe hole carrier range is severely degraded to prevent the injection ofholes from the biasing electrode 14 into the bulk selenium layer 18,without altering significantly the electron conduction properties ofthis layer. This alkali doped selenium buffer layer 22 may also includearsenic in the amount of 0.5-5 wt %.

FIG. 2(a) illustrates a cross section of test devices that were built toapproximate reduced size X-ray detectors, and specifically to verify theadvantages of the present invention. The same reference numbers as inFIG. 1 are used to identify the same elements. As shown in FIG. 2(a), aglass Corning 7059™ substrate 26, 1.1 mm thick, was coated with aconductive transparent indium tin oxide (ITO) layer 28 known asBaltracon™ and supplied by Balzers. For the sake of comparison, half ofthe substrate was masked off prior to the evaporation of the amorphousarsenic triselenide layer 20. A thin film of a-As₂Se₃, having athickness of 3 μm, was formed by thermally evaporating the doped arsenictriselenide material under reduced pressure of less than 1×10⁻⁵ torr.The composition of the precursor material was 34-38 wt % As, andincluded other dopants, such as iodine, indium or gallium in parts permillion concentration. The material was evaporated from a stainlesssteel 304 boat which was held at a temperature ranging from 350-450° C.The doped arsenic triselenide vapour was condensed onto the portion ofthe substrates 26, 28 provided for it and held at a temperature between30 and 190° C. Under the above conditions, a uniform, amorphous,pin-hole free buffer layer was obtained over the half of the substratewhich was not masked. The doped amorphous selenium layer 18 between 200and 500 μm thick, which serves herein as the X-ray energy absorbing andcharge converting layer, was then evaporated from a second stainlesssteel boat held between 230 and 280° C. onto the above substrates afterthe mask was removed. The temperature of the substrates was heldconstant between 50 and 90° C. during the deposition of this secondlayer. An insulating dielectric layer 30 of a thickness between 30 and150 μm, for example of polycarbonate, polyester or parylene, was thenlaminated or coated onto the amorphous selenium photoconductive layer.The prototype configuration was then completed by evaporating a thin(10-50 nm) transparent conductive material such as gold, platinum,aluminum or indium tin oxide in a patterned form onto the dielectricmaterial to create two independent electrodes 32, 34. These topelectrodes were deposited in locations such that a comparative analysiscould be performed to illustrate the role of the amorphous arsenictriselenide buffer layer 20 with reference to the prior art.

In a further embodiment of the present invention, illustrated in FIG.2(b), an arsenic and sodium doped selenium alloy buffer layer 22 wasevaporated onto the doped amorphous selenium thick film 18 on the samehalf of the substrate as layer 20 in FIG. 2(a) which itself was omitted.This was done from a stainless steel boat held at a temperature between230 and 270° C. The substrate temperatures during this evaporation wereheld between 50 and 80° C. No dielectric insulating layer 30 was neededin this case and only the two independent electrodes 32, 34 wereprovided.

To illustrate the first key element of this invention relating to thereduction of interfacial crystallization, the samples as prepared abovein FIG. 2(a) were analyzed by infra-red microscopy techniques. Theresults from this study showed that the inclusion of the amorphousarsenic triselenide layer 20 greatly reduced the microcrystallites whichnormally grow at the interface between the selenium layer 18 and thesubstrate materials 26, 28 (or 12 in FIG. 1) during the deposition ofthe doped amorphous selenium layer in detectors fabricated in accordancewith the prior art. With the presence of an amorphous arsenictriselenide layer 20, in this case having a thickness of 3 μm, noevidence of any crystallites could be observed, while on the other sideof the detector, with no buffer layer, severe crystallization wasobserved at the interface. This key element of the invention leads to asuperior performing X-ray detector, void of any crystallite-inducedimaging artifacts.

Another important element of this invention provides for improvedmechanical stability of the novel multilayered X-ray detector or plate.Such detector may, under special circumstances, such as shipping duringcold weather conditions, be subjected to temperature cycling. Prior tothis invention, temperature cycling Seriously affected the mechanicalintegrity of the device due to a mismatch in the thermal expansioncoefficients of the substrate material and the photoconductive seleniumlayer. Physical delamination of the selenium layer or selenium filmbreakage would sometimes occur. FIGS. 3(a) and 3(b) illustrate how theselenium substrate interface may be stressed by differential thermalexpansion upon cooling (tensile stress) or heating (compressive stress)of the device. Thus, in FIG. 3(a) an illustration is provided how theinterface between substrate 12 and doped selenium layer 18 may besubjected to a compressive stress and in FIG. 3(b) to a tensile stressdue to temperature cycling.

In FIGS. 3(c), 3(d) and 3(e) the various failure mechanisms areillustrated, which occur if the shear stress is not reduced. Thus, inFIG. 3(c) a delamination of the doped selenium layer 18 from thesubstrate 12 is shown, in FIG. 3(d) breaks or cleavages 36 occur in thedoped selenium layer 18, and in FIG. 3(e) cracks 38 occur in thesubstrate 12 as a result of the stresses shown in FIGS. 3(a) and 3(b) ofFIG. 3.

Amorphous selenium layers are known to have very large thermal expansioncoefficients, comparable to polymers. On the other hand, materialstypically used as substrate materials, for example aluminum or glass,have thermal expansion coefficients much lower than the amorphousselenium. The thermal expansion of selenium-arsenic alloys has beenfound to be strongly related to the arsenic concentration. The use of anamorphous arsenic triselenide buffer layer decreases the shear stress atthe interface inasmuch as the thermal linear expansion coefficient ofthis buffer layer is more than two times lower than that of the dopedamorphous selenium layer and more closely matches that of the substratematerial. The impact of this is that the stress is thus transferred fromthe relatively weak interface between the substrate material and thedoped amorphous selenium film to the much stronger interface between thedoped amorphous selenium layer and the amorphous arsenic triselenidebuffer layer, thereby increasing the film adherence.

In order to illustrate the feature of the present invention relating tothe ability of the arsenic triselenide buffer layer to reduce the stressresulting from the mismatched thermal expansion between the thickamorphous selenium layer and the substrate, the following experiment wasperformed: two test plates were purposively built, one such as shown inFIG. 2(a) including the thin buffer layer 20 of amorphous arsenictriselenide, and the other excluding such buffer layer 20. Thedeposition process of the various layers has already been describedabove. Thereafter, these two test plates were subjected to a thermalcycling and a conventional Tencor FLX-2900™ film stress measurementinstrument was used to record their respective stress-temperaturecharacteristics. The obtained experimental results plotted in FIG. 4show that for a given temperature difference, the plate with the bufferlayer presents a stress change of about five times lower that of theplate without the buffer layer. This result indicates that the platewith the buffer layer is much less temperature-sensitive. Thus, as faras the mechanical integrity of the plate is concerned (during handling,storage and shipping), the use of the buffer layer would allow themultilayered detector to withstand thermal shocks since thecorresponding thermally-induced stress values would be still low enoughnot to lead to any plate failure mechanisms illustrated in FIGS. 3(c),3(d) and 3(e).

For selenium-based X-ray imaging systems designed specifically forsnap-shot mode of operation, a reconditioning sequence is alwaysrequired between two successive X-ray image acquisitions, to eliminatethe space charge accumulated in the selenium layer which represents thelatent electrostatic X-ray image. Prior to this invention, this imageerasure step was accomplished through a series of light, bias voltagesequences and sometimes temperature cycling. This external erasuretechnique is usually cumbersome and time consuming. The presentinvention provides a multilayered detector structure which is capable ofeliminating any residual space charge which accumulates in the dopedamorphous selenium layer and thus it is self-reconditioning.

To illustrate this feature of the present invention, a conventionalTime-of-Flight Transient Photoconductivity apparatus was used to probethe internal space charge distribution in the prototype sample asdescribed above and shown in FIG. 2(a) after it was subjected to atypical X-ray imaging sequence. In the Time-of-Flight measurement, ahigh voltage bias was applied across the X-ray detector prototype togenerate a uniform electric field within the selenium layer. A shortduration (200 picosecond) highly absorbed (λ=460 nm) light pulsegenerated from a nitrogen pumped dye laser was focussed on the top metalelectrodes 32, 34 of the sample causing the photogeneration of chargesnear the upper surface of the selenium layer. Due to the polarity of theapplied bias voltage, holes were swept to the bulk of the selenium layercausing a measurable current to flow in the external circuit. In theabsence of any charge trapping or any perturbation of the internalelectric field by space charge, this current should be constant inmagnitude until the holes reach the counter bottom electrode, at whichtime the measured current should abruptly fall to zero.

FIG. 5(a) shows the results on a device which did not include theamorphous arsenic triselenide layer 20, whereas FIG. 5(b) shows theresults on a device including this buffer layer. In these figures, whenreference is made to E−6, it means the number is to the exponential of−6. The first hole photocurrent waveform shown in FIG. 5(a) and FIG.5(b) demonstrates the absence of any noticeable space charge in eitherdevice after prolong dark resting. Prior to the acquisition of thesecond hole photocurrent waveform, the devices were X-ray irradiated tosimulate an X-ray imaging sequence. The second photocurrent waveforms inFIG. 5(a) and FIG. 5(b) indicate the presence of a significant X-rayinduced space charge accumulated at the interface between the seleniumlayer and the dielectric material in both devices. It should be notedthat this space charge would normally represent the latent electrostaticimage in an actual X-ray imaging plate. The third hole current waveformsin FIG. 5(a) and FIG. 5(b) were obtained after briefly shorting theupper and bottom electrodes together. The complete restoration of thehole photocurrent waveform to its space charge neutral condition in FIG.5(b) demonstrates the self-reconditioning capability provided by theamorphous arsenic triselenide layer in accordance with this invention.In marked contrast, the device without the arsenic triselenide layer 20could not recondition itself as evidenced by the third current holewaveform in FIG. 5(a). The self reconditioning aspect of this inventionis caused by the amorphous arsenic triselenide layer 20 which acts likea hole reservoir. When the applied bias voltage is removed across thedevice, the negative space charge at the selenium dielectric interfacecauses the internal field in the selenium layer to actually reversepolarity. This internal field reversal draws-in positively charged holesfrom the buffer layer 20 until the internal field drops to zero, atwhich point the device is returned to its space charge neutral state.The hole reservoir effect of the amorphous arsenic triselenide bufferlayer 20, which was discovered by the applicants is actually caused bytwo effects: amorphous arsenic triselenide is a higher conductivitymaterial (10⁻¹⁵ Ω⁻¹cm⁻¹), and this increased conductivity is attributedto an increased number of free holes in the material. In addition, thejunction between the amorphous arsenic triselenide material and thesubstrate electrode forms a “finite injector” which can supply enoughcharge to neutralize the negative space charge.

To illustrate a further important element of this invention, deviceswere prepared as described above except without the aforementioneddielectric and metal electrode layer. A key feature of a goodselenium-based X-ray detector is to exhibit a low dark discharge currentwhen biased to a high electric field. One of the main components of thisdark current is electron injection from the conducting substratematerial into the bulk amorphous selenium layer. Good detectors must,therefore, be designed to minimize this process. In order to demonstratethis key element of the present invention, test devices were testedxerographically by approximating electron injection from the substratematerial through a photogeneration process. The prototype devices werefirst charged by a corona device so that the bare surface of theselenium layer was biased positively with respect to the substrate. Thesurface potential of the selenium layer was then monitored by anon-contact electrostatic probe for a period of three hundred seconds toevaluate the dark discharge characteristics of the device. The samplewas then recharged by the corona device prior to approximating severeelectron injection from the substrate material by illuminating thedevice through the transparent substrate material. FIG. 6(a) shows thexerographic results on a sample which did not include the amorphousarsenic triselenide layer. Since the surface potential drops drasticallyduring the time where light was illuminated through the substrate due tothe transport of photoinjected electrons across the selenium layer, thisillustrates that any spurious charge injection from the substratematerial into the selenium layer will cause large local dark currents.FIG. 6(b) shows the same xerographic test performed on a prototypedevice which includes the interstitial arsenic triselenide layer. Inthis case, however, the illumination of the device by the highlyabsorbed light did not cause any appreciable discharge of the devicedespite the fact that the amorphous arsenic triselenide material isphotosensitive to the wavelength of the light used. This result issurprising and is interpreted by the fact that electrons photogeneratedin the arsenic triselenide layer become deeply trapped and are unable todischarge the device even though the buffer layer is very thin. Thisstudy demonstrates the fact that the inclusion of the amorphous arsenictriselenide buffer layer makes the X-ray detector immune to spuriouselectron injection from the substrate material inasmuch as the injectedcharge will become deeply trapped immediately in the arsenic triselenidematerial.

Finally, another feature of this invention relates to the requirement insome X-ray imaging systems to acquire images at video rates (30 imagesper second). The X-ray beam on-time is typically several minutes in thisimaging mode which is called the dynamic mode imaging. Due to the finiteelectron range of doped amorphous selenium, typically 10⁻⁵ cm²/V, a netnegative space charge will accumulate in the selenium layer due to thelong imaging exposure time and the superior transport properties of theholes. This negative space charge does not adversely affect the imagingperformance provided that it remains constant for a time scale muchlonger than the imaging time scale. The negative space charge can changebecause of two processes—thermal detrapping of the electrons andsubsequent sweep out or injection of holes from the upper biasingelectrode and charge recombination. For selenium, the former process isunlikely inasmuch as the electron trap depth is 1.1 eV, leading to athermal release time constant of several hours for the trappedelectrons. A top electrode structure which minimizes hole injection istherefore required in this mode of operation. The use of a top unipolarconducting buffer layer, sandwiched between the doped amorphous seleniumlayer and the top biasing electrode, can thus significantly reduce thetop hole injection into the bulk of the doped amorphous selenium layer.

For this purpose, an alkali doped thin selenium buffer layer with a holerange of less than 10⁻¹⁰ cm ²/V is directly deposited on the bipolartransport photoconductive selenium layer. A top conducting electrode isthen deposited on top of this buffer layer. The extremely short holerange of the top buffer layer ensures that any hole injected from thetop electrode will be trapped in this layer, thus preventing it fromrecombining with the negative space charge in the bulk selenium layer.This enables the negative space charge to be unaltered during the X-raybeam on-time. Under normal X-ray conditions where the top electrode ispositively biased with respect to the bottom electrode, the electronsand holes generated in the amorphous selenium layer should both reachthe top and bottom electrodes without being trapped. Thus, the thinalkali doped buffer layer just also not inhibit the flow of X-raygenerated electrons to the top electrode. It has been shown, in thisregard, that doping of selenium with Li, Na, and K severely degrade onlythe hole range, leaving the electron range relatively unaffected.Similarly, the aforementioned amorphous arsenic triselenide buffer layeris designed in such a way as to maximize the hole range, in order toallow the X-ray generated holes to reach the bottom electrode.

FIG. 7 shows a plot of the dark current-voltage characteristics of adevice with a unipolar buffer layer 22 made of Na-doped selenium film asshown on the left hand side of FIG. 2(b), and without such layer 22 asshown on the right hand side of FIG. 2(b). The biasing electrodes 32, 34consisted, in this case, of Pd. It can be seen from the graph in FIG. 7(where pA means picoamperes) that the two devices behave quitedifferently at high electric fields, with the device that has nounipolar buffer layer having a much greater dark current. This isattributed to field-assisted emission of holes from the Pd electrode 34into the selenium film 18. Once injected, these excess holes contributeto the large dark current which greatly exceeds the intrinsicresistivity of the device. On the other hand, the device with theunipolar conducting buffer layer 22 situated between the bias electrode32 and the doped amorphous selenium film 18 has a much lower darkcurrent at the same electric field. This is caused by the fact that anyfield-assisted injection of holes from the electrode material isabruptly stopped because these holes immediately get trapped in theNa-doped material. Once trapped, these holes actually lower the electricfield at the electrode interface, which also tends to lower the holeinjection rate.

To further illustrate how the unipolar Na-doped buffer layer isbeneficial in fluoroscopy, a simple test was performed where the totalcurrent flowing from the device was measured in the presence of a pulsedX-ray beam with a total beam on-time of about 30 seconds. FIG. 8(a)shows the total current (dark current +X-ray current) as a function oftime in the device without the unipolar buffer layer, whereas FIG. 8(b)shows the total current as a function of time in the device with theunipolar buffer layer. For the device having no unipolar buffer layer,it is clear that the dark current, as measured during the intervalbetween the X-ray pulses, increases in value during the presence of theX-rays, and takes more than 30 seconds to return to its previous valuebefore the application of the X-rays. This enhanced dark current is dueto hole injection from the top electrode to neutralize the X-raygenerated negative space charge in the selenium layer. In an imagingmode such as fluoroscopy, this dark current recovery time manifestsitself in the form of an image lag inasmuch as an excess current stillpersists in that region of the detector long after the primaryphotocurrent has expired.

FIG. 8(b) shows the results of the same test performed on the devicewith the unipolar buffer layer. Here it is clear that the dark currentremains stable during the application of the X-rays. When the finalX-ray pulse expires, the current drops abruptly to the device's darkcurrent value that was there before the X-ray exposure began. This noveldevice does not exhibit the detrimental image lag that the previousdevice demonstrated.

In FIGS. 8(a) and 8(b), reference to E−9 means that the number is to theexponential of −9.

It should be understood that the invention is not limited to thespecific embodiments described above, but that many modificationsobvious to those skilled in the art can be made without departing fromthe spirit of the invention and the scope of the following claims.

What is claimed is:
 1. A method of manufacturing a multilayer X-rayimaging plate which comprises: (a) providing a plate of a substrate; (b)depositing on said substrate a thin film of amorphous arsenictriselenide by thermally evaporating doped arsenic triselenide materialunder reduced pressure of less than 1×10⁻⁵ torr and condensing theresulting vapour onto the substrate to form a uniform amorphous layer ofarsenic triselenide; (c) depositing on said thin film of amorphousarsenic triselenide a thick photoconductive film of doped amorphousselenium by evaporating a doped amorphous selenium material andcondensing the resulting vapour onto said thin film of amorphous arsenictriselenide; and (d) laminating or coating directly onto the thickphotoconductive film an insulating dielectric layer and providing on topof said insulating dielectric layer a thin layer of conductive material,said insulating dielectric layer and said layer of conductive materialforming a biasing electrode.
 2. A method according to claim 1, in whichthe substrate is selected from the group consisting of aluminum, glass,a thin film transistor array, a charged coupled device and acomplementary metal oxide semiconductor device.
 3. A method according toclaim 1, in which said insulating dielectric layer is formed ofpolycarbonate, polyester or parylene, and said thin layer of conductivematerial is formed of gold, platinum, aluminum, chromium, indium orindium thin oxide.
 4. A method according to claim 1, in which saidphotoconductive film is deposited to a thickness of between about 100 μmand 2 mm.
 5. A method according to claim 1, in which saidphotoconductive film is deposited to a thickness of between about 200 μmand 500 μm.
 6. A method according to claim 1, in which the thin film ofamorphous arsenic triselenide is obtained from a precursor materialcontaining 34-38% As and dopants selected from the group consisting ofiodine, indium and gallium in parts per million concentration.
 7. Amethod according to claim 6, in which the thin film of amorphous arsenictriselenide is deposited to a thickness of about 0.5 μm-10 μm.
 8. Amethod according to claim 6, in which the thin film of amorphous arsenictriselenide is deposited to a thickness of about 1 μm-5 μm.
 9. A methodof manufacturing a multilayer X-ray imaging plate which comprises: (a)providing a plate of a substrate selected from the group consisting ofaluminum, glass, a thin film transistor array, a charged coupled deviceand a complementary metal oxide semiconductor device; (b) depositingdire on said substrate a thick photoconductive film of doped amorphousselenium by evaporating doped amorphous selenium material and condensingthe resulting vapour onto said substrate; (c) depositing on saidphotoconductive film a thin film of alkali doped selenium by evaporatingan alkali doped selenium alloy or co-evaporating Se and an alkalimaterial, and condensing the resulting vapour onto the photoconductivefilm of doped amorphous selenium; and (d) forming a conducting biasingelectrode on top of said film of alkali doped selenium.
 10. A methodaccording to claim 9, in which the biasing electrode is formed of a thinconducting layer of a material selected from the group consisting ofgold, platinum, aluminum, chromium, indium and indium tin oxide.
 11. Amethod according to claim 9, in which the thin film of alkali dopedselenium is doped with Li, Na, or K and also contains 0.5-5 wt % of As.12. A method of manufacturing a multilayer X-ray imaging plate whichcomprises: (a) providing a plate of a substrate; (b) depositing on saidsubstrate a thin film of amorphous arsenic triselenide by thermallyevaporating doped arsenic triselenide material under reduced pressure ofless than 1×10⁻⁵ torr and condensing the resulting vapour onto thesubstrate to form a uniform amorphous layer of arsenic triselenide; (c)depositing on said thin film of amorphous arsenic triselenide a thickphotoconductive film of doped amorphous selenium by evaporating a dopedamorphous selenium material and condensing the resulting vapour ontosaid thin film of amorphous arsenic triselenide; (d) depositing on saidphotoconductive film a thin film of alkali doped selenium by evaporatingan alkali doped selenium alloy or co-evaporating Se and an alkalimaterial, and condensing the resulting vapour onto the photoconductivefilm; and (e) forming a suitable biasing electrode directly on top ofsaid thin film of alkali doped selenium.